Hushkits are designed to reduce
low bypass ratio turbofan engine noise. Two primary sources of engine noise are:
The high pitched whine generated by the first stage fan blades
passing through the wakes of the stationary inlet guide vanes.
The characteristic rumble caused by the high velocity jet plume.
The DC-9 Hushkit, LLC Stage III Hushkit reduces jet noise by installing a
twelve lobe Pratt & Whitney mixer. The mixer reduces jet noise by efficiently mixing
the cool bypass fan air with the hot core exhaust. This helps reduce the primary source of
jet noise, the high velocity core jet. To accommodate the mixer the nozzle exit area of
the thrust reverser is increased.
DC-9 series aircraft with take-off gross weights greater than 91,000
pounds incorporate both the 12 lobe mixer and the Pratt & Whitney Respaced Inlet Guide
Vanes (RIGV). The Respaced Inlet Guide Vane increases the axial spacing by five inches
between the inlet guide vanes and the first stage fan. The additional spacing between the
inlet guide vanes and the first stage fan helps reduce the forward propagated fan noise.
The upper and lower cowl doors are modified to accommodate the five inch extension of the
inlet guide vanes.
Two other features of the DHC Stage 3 Hushkit involve the aircraft wing
flaps and the engine CSD Oil Cooler:
Stage 3 noise operation is limited, on approach, to 40-degree flaps.
For the right side engine, the CSD oil cooler is relocated from the
outboard side of the engine to the inboard side.
What is Stage 3?
